1) Having a loot midget unexpectedly pop out of a loot chest and scaring the crap out of you when you are not prepared for it. Thankfully that was an achievement anyway, so that made it worth the repeated shock.

2) Realizing that while you can pick up the zombie brains through the ENTIRE Dr. Ned's DLC, you actually need to acquire the brain collecting mission before they will even count. Learned that one the hard way.